New Zealand Prime Minister Jacinda Ardern Pregnant

New Zealand Prime Minister Jacinda Ardern has announced she is pregnant and will give birth in June.

In an official statement on Friday, Ms Ardern spoke of the joy of she and her partner Clarke Gayford.

"We're both really happy. We wanted a family but weren't sure it would happen for us, which has made this news unexpected but exciting," she said.

Deputy Prime Minister Winston Peters will take on the top job for six weeks after the baby is born.

"As is the case when I am overseas, Mr Peters will act as prime minister, working with my office while staying in touch with me. I fully intend to be contactable and available throughout the six week period when needed," Ms Ardern said.

"At the end of my leave I will resume all prime ministerial duties."

The couple found out on October 13 - in the midst of negotiations over who would form the new government - but had kept it quiet as was common in the early stages of pregnancy, she said.

Ms Ardern said she and Mr Gayford were privileged enough to allow him to stay at home as a stay-at-home dad.

"Knowing that so many parents juggle the care of their new babies, we consider ourselves to be very lucky."

The couple received congratulations from across the political spectrum following the announcement - including from former NZ prime ministers Bill English and Helen Clark - and prompted more than 40,000 tweets in an hour.

In August, Ardern was grilled about baby plans on the radio, comments she called ‘totally unacceptable’.

One of the panellists on the program, Mark Richardson had said at the time: “If you are the employer of a company you need to know that type of thing from the woman you are employing ... the question is, is it OK for a PM to take maternity leave while in office?"

Arden clapped back.

"For other women, it is totally unacceptable in 2017 to say that women should have to answer that question in the workplace. That is unacceptable in 2017. It is the woman's decision about when they choose to have children," she said.

"It should not predetermine whether or not they get the job."

Ms Ardern is not the first world leader to be pregnant. Pakistan's Benazir Bhutto caused a stir with her second pregnancy in 1990.